Soybean oil (SBO) was modified with polystyrene via a radical graft polymerization reaction for use as a processing oil in tire tread compounds. Poly(styrene-butadiene)/polybutadiene rubber compounds with silica and carbon black, containing different processing oils including naphthenic oil (NO), aromatic oil (AO), SBO, and polystyrene-modified SBO (SBO-PS), were formulated, vulcanized, and tested. The curing behavior, mechanical properties, and dynamic properties were investigated. The cure test results showed that all SBO-based rubbers had a shorter scorch time and cure window than the NO- and AO-based rubbers.
The tensile tests demonstrated that partial and complete replacement of NO with SBO led to reduced tensile modulus but increased elongation of rubber. For the rubbers compounded with SBO-PS and with a 50/50 mixture of NO/SBO-PS, tensile strength and elongation were higher than for the NO-based rubber. The same tendency was observed when SBO-PS–based rubbers were compared with SBO- and AO-based rubbers. SBO-PS–based rubbers demonstrated better tensile properties than AO-based rubbers and far better properties than SBO-based rubbers. In the tear resistance test and durometer hardness test, SBO-PS contained rubbers that showed similar properties to NO-containing rubber.
The dynamic mechanical analysis of SBO-PS–containing rubbers demonstrated that use of this compound in tire treads is expected to improve both rolling resistance and wet traction when compared with an AO-based rubber. The modification of SBO with grafted PS is a promising method of making processing oil, which can replace petroleum-based processing oils with bio-based renewable oils in tire tread compounds while improving their properties.